Influence of heat treatment in sintering process on characteristics of A[l.sub.2][o.sub.3]-Zr[o.sub.2] ceramics systems

American Journal of Applied Sciences, Feb, 2009 by A. Rittidech, T. Tunkasiri

INTRODUCTION

The most widely used ceramic materials are alumina, A[l.sub.2][O.sub.3] and zirconia, Zr[O.sub.2], because of their excellent bio-compatibility. The main advantage of A[l.sub.2][O.sub.3] is its high hardness and wear resistance, while Zr[O.sub.2] exhibits higher strength and fracture toughness, besides its lower Young' s modulus (1), (2), (3), (4), (5). Tetragonal zirconia in alumina matrix is known as Zirconia Toughened Alumina (ZTA). ZTA is a high purity combination of the low cost of alumina and high strength of zirconia. Moreover, its is a ceramic-ceramic composite with good mechanical properties as shown by Aruna and Rajam (6). Then ZTA ceramics are attractive materials due to the combination of both Zr[O.sub.2] and A[l.sub.2][O.sub.3] properties. The stoichiometry of ZTA is known to be an important factor for ensuring phase composition, mechanical properties and microstructure characteristics. To obtain stoichiometric ZTA, different preparative method have been introduced, such as hydrothermal (7), mixed oxide (8), and gel casting (9).All these techniques are aim to improve properties of A[l.sub.2][O.sub.3]-Zr[O.sub.2] ceramics. Recently, some researchers have focused their attention on optimal addition Zr[O.sub.2] into A[l.sub.2][O.sub.3] to enhance good mechanical property (10). Moreover, they have found that the sinterability of the ceramics matrix is reduced when a large amount of second phase is added (11). Therefore, in the present work focus on A[l.sub.2][O.sub.3]-Zr[O.sub.2] ceramics systems difference stoichiometry which are prepared using solid state reaction of mixed oxide route. The effect of heating/cooling rates in sintering conditions on densification, grains size and hardness are investigated in this connection.

EXPERIMENTAL PROCEDURE

The A[l.sub.2][O.sub.3]-Zr[O.sub.2] ceramics with (1-x) A[l.sub.2][O.sub.3]-xZr[O.sub.2] where x = 0.5, 0.15, 0.25, 0.35, 0.45 and 0.50 were prepared from A[l.sub.2][O.sub.3] and Zr[O.sub.2] as precursors and isopropyl alcohol as solvent. All the six different batches were then ball milled with Zr[O.sub.2] media under isopropyl alcohol for 24 h. After ball-milling for 24 h, drying in electric furnaces, the resulting powders were calcined for 2 h at 1100[degrees]C with 5[degrees]C [min.sup.-1]heating/cooling rate. Powders were uniaxial pressed at 3 MPa to form pellets. Sintering temperature was done at 1600[degrees]C for 2 h with heating/cooling rates from 1-10[degrees]C [min.sup.-1]. The bulk densities of sintered sample were calculated using Archimedes's method. Microstructural analysis was examined by using Scanning Electron Microscopy (SEM) and Energy-Dispersive X-ray spectrometry (EDX) (JEOL JSM840A) on a polished surface of sintered samples. The average grain sizes of the ceramics were determined using the linear intercept method as suggested by Lee and Rainforth (12). Hardness of bulk ceramics were measured using a microscan OD Vickers (Model MAT 24 Brooks).

RESULTS AND DISCUSSION

The densification data of sintered samples are shown in Table 1. Table 1 contains the information of densities of the ceramics. In general, the bulk density was found to slightly increase with heating/cooling rate but the densities were obviously tend to increase with x contents, which could be due to Zr[O.sub.2] concentrations.

Table 1: Density of the (1-x) [Al.sub.2][O.sub.3]-
xZr[O.sub.3] ceramics from sintered various heating/cooling
rates

Heating/        Densities (g [cm.sup.-3])
cooling rates
[degrees]C/min  x=0.05   x=0.15  x= 0.25  x=0.35  x=0.45  x=0.50

1                3.97     4.13    4.27     4.45    4.65    4.84
3                4.15     4.16    4.31     4.46    4.58    4.71
5                4.01     4.18    4.25     4.49    4.62    4.82
7                4.02     4.14    4.31     4.48    4.64    4.72
10               4.24     4.21    4.32     4.45    4.68    4.75

It is observed that a density of between 3.97 and 4.84 g [cm.sup.-3] SEM micrographs of selection ZTA ceramics are shown in Fig. 1. In general, similar microstructural characteristics were observed in these samples, i.e., uniformly sized grains with a high degree of grain close-packing. By applying the linear intercept method as suggested by to these SEM images, grain sizes were estimated for these samples as given in Fig. 2.

[FIGURE 1 OMITTED]

[FIGURE 2 OMITTED]

It is obviously seen that heating/cooling rates are the important parameters for the development of ceramic microstructures. In that the average grain size decreases with increasing heating/cooling rate. It may be assumed that, the short heating in sintering process were inhibited growth of grain then in slow heating/cooling rates were obtained a large grains. Further increase in Zr[O.sub.2] contents lead to decrease in average grain size. This results indicates that Zr[O.sub.2] is to reduce the grain growth in A[l.sub.2][O.sub.3]-Zr[O.sub.2] ceramics system and to improve the homogeneity of microstructure and being consistent with literatures(13), (14).